1. Field of the Invention
The present invention relates to an image forming apparatus, such as a copying machine and a printer, having a plurality image forming portions in which an image is formed by an electrophotographic system.
2. Description of the Related Art
An image forming method in a color image forming apparatus in which a multiple intermediate transfer method is used will be described with reference to FIG. 10.
Four process units which are of electrophotographic-system image forming portions are provided according to yellow, magenta, cyan, and black colors.
In FIG. 10, there are shown photosensitive drums 1a to 1d, charging means 2a to 2d, exposure means 3a to 3c, development means 4a to 4d, an intermediate transfer belt 51, primary transfer members 53a to 53d, and photosensitive drum cleaners 6a to 6d. In addition, secondary transfer members 56 and 57, and an intermediate transfer belt cleaner 55 are also shown. After the photosensitive drums 1a to 1d are uniformly charged with the charging means 2a to 2d, the exposure means 3a to 3d perform exposure according to an image signal, thereby forming electrostatic latent images on the photosensitive drums 1a to 1d. Then, the development means 4a to 4d develop toner images, and transfer high-voltage power supplies 34a to 34d apply transfer biases to the transfer members 53a to 53d, which allows the toner images on the photosensitive drums 1a to 1d to be sequentially transferred to the intermediate transfer belt 51.
The images sequentially multiple-transferred onto the intermediate transfer belt 51 from the photosensitive drums 1a to 1d are transferred to a recording material P by applying a secondary transfer bias between secondary transfer members 56 and 57. The toner image on the recording material P is fixed by a fixing device 7 to obtain a full-color image.
The intermediate transfer belt having a self-attenuation property whose surface resistivity is lower than about 1013 Ω/□ (with a probe conformable to JIS-K6911, at applied voltage of 100 V for an applied time of 60 sec, at 23° C. and 50% RH) is frequently used in the image forming apparatus in which the multiple intermediate transfer method is used.
When the images are sequentially transferred from the plurality of process units using the intermediate transfer belt having the above property, because the applied voltage is not sequentially increased, it is not necessary to enlarge the transfer high-voltage power supply. Additionally, in the intermediate transfer belt having the above property, a discharge phenomenon is seldom generated in a roller portion about which the intermediate transfer belt is entrained, and the intermediate transfer belt is easy to use.
Like the image forming apparatus including a mode in which only the black toner image is formed, in the image forming apparatus in which the photosensitive drums differ from one another in an operation frequency, the photosensitive drums having different lifetimes are used according to the operation frequency of each process unit.
At this point, sometimes the photosensitive drums having the different lifetimes differ from one another in properties during the exposure.
As a result, in charging the photosensitive drums 1a to 1d with the charging means 2a to 2d, the photosensitive drum charged with the same polarity (for example, negative polarity) as charged toner and the photosensitive drum charged with an opposite-polarity (for example, positive polarity) are used while mixed.
Sometimes an image portion in which the exposure is performed by Image Area Exposure (IAE) and an image portion in which the exposure is performed by Background Area Exposure (BAE) are mixed.
A: In the image area exposure method, the photosensitive drum is charged with the negative polarity to expose and develop the image area. The transfer bias having a polarity opposite the toner and a predetermined potential difference (transfer contrast voltage) with a drum potential at the image portion is applied to transfer the toner image from the photosensitive drum to the intermediate transfer member.
B: In the background area exposure method, the photosensitive drum is charged with the positive polarity, the background area is exposed, and the non-exposed portion is developed. The transfer bias having a polarity opposite the toner and the predetermined potential difference (transfer contrast voltage) with the drum potential at the image portion is applied to transfer the toner image from the photosensitive drum to the intermediate transfer member.
FIG. 11 shows a relationship between the photosensitive drums 1a to 1d and the potential at the transfer member when the toner charged in the negative polarity is developed.
There are the process unit in which the photosensitive drum is charged with the negative polarity using the negative toner to form the image by the image area exposure method as shown in FIG. 11 and the process unit in which the photosensitive drum is charged with the positive polarity to form the image by the background area exposure method. It is found that the following problems are generated when the same image forming apparatus is provided with the two different process units to adopt the image multiple transfer method.
That is, as shown in FIG. 11, when the toner image on the photosensitive drum is transferred, the transfer bias having the predetermined potential difference (transfer contrast voltage) is applied to the transfer member. At this point, in the process unit of A: image area exposure method, the photosensitive drum surface is with the negative polarity. Therefore, the toner image with the negative polarity can be transferred by applying a relatively small positive voltage as a transfer bias Va.
On the other hand, in the process unit of B: background area exposure method, the photosensitive drum surface is with the positive polarity. Therefore, the voltage higher than the transfer bias Va is applied as a transfer bias Vb to obtain the predetermined potential difference.
The potential difference is increased between the process unit of the image area exposure method and the process unit of the background area exposure method, in the image forming apparatus in which the process unit of the image area exposure method and the process unit of the background area exposure method are next to each other. That is, the potential difference becomes extremely large between a negative surface potential of the photosensitive drum in the process unit of the image area exposure method and the positive transfer bias Vb applied to the transfer member of the background area exposure method. This causes a phenomenon called “interference” as described below. A transfer current flows to an adjacent image forming portion from a primary transfer member of an image forming portion, which creates lack of the transfer current to generate defective image. Once the transfer bias of the adjacent transfer member is turned on or off, a change in transfer current is generated to form a step in image density.
The transfer biases of the adjacent process units are set to have close values, in the image forming apparatus in which only the negatively charged process unit of the image area exposure method is used and the image forming apparatus in which only the positively charged process unit of the background area exposure method is used. Therefore, only a minor interference phenomenon is generated.
However, as described above, it is difficult to avoid an interference phenomenon in the image forming apparatus in which the negatively charged photosensitive drum and the positively charged photosensitive drum are simultaneously used. Unfortunately, once the transfer bias of the adjacent transfer member is turned on or off, the change in transfer current is generated to form the step in the image density.